The present invention relates to the field of the industrial manufacture of acetic acid and/or methyl acetate.
It relates more particularly to a continuous process for the manufacture of acetic acid and/or methyl acetate during which the composition of the catalyst system is modified in a particularly smooth and uniform manner, making it possible, in said industrial process, to change from a catalyst system based on rhodium alone to a catalyst system based on rhodium, and iridium, or even based on iridium alone, without having to shutdown the production unit in order to effect this change of catalyst.
Numerous industrial processes are known for the manufacture of acetic acid and/or methyl acetate in the liquid phase under pressure using a homogeneous catalyst system.
The invention applies quite specifically to the modification of processes which initially involve the carbonylation of methanol or a carbonylatable derivative of methanol in the presence of a rhodium-based catalyst.
The patents of Paulik et al., which form the basis of the so-called “Monsanto” technology or process, describe the carbonylation of reactants in the liquid or vapor phase by homogeneous or heterogeneous catalysis with rhodium (U.S. Pat. No. 3,769,329) or iridium (U.S. Pat. No. 3,772,380). In this technology the reactants consist of alcohols ROH, ethers R′—O—R′, esters R′C(O)OR′ and halides R—X in which the total number of carbons is less than or equal to 20. The carbonylation reaction is carried out in the presence of one of these reactants, carbon monoxide (CO partial pressure of between 1 and 15,000 psig), a catalyst system comprising rhodium or iridium in the presence of a halogenated promoter, and water, at a temperature of between 50 and 300° C.
The first carbonylation processes to be developed were catalyzed by a rhodium compound in the presence of a methyl halide, preferably methyl iodide, as co-catalyst. These processes were regularly improved to increase the reaction rates (and hence the productivity of the process), decrease the production of impurities, increase the stability of the catalyst in solution and reduce the manufacturing costs.
In particular, it has been possible to reduce the water content by adding large amounts of iodide ions to the reaction medium (more than 0.3 mol/liter in patent FR 2 551 434) and/or by maintaining imposed concentrations of the constituents of said reaction medium (EP 0 161 874 and EP 0 250 189).
Recently, studies performed with a view to replacing rhodium with iridium have culminated in novel so-called “low water content” processes catalyzed by iridium alone (EP 0 618 184, EP 0 616 997, EP 0 786 447) or by iridium in the presence of co-promoters, mainly ruthenium, osmium or rhenium (EP 0 643 034, EP 0 728 729,EP 0 752 406).
Conventionally, these various processes of carbonylation in the liquid phase under homogeneous catalysis are carried out in an installation comprising 3 separate zones. In the 1st zone, called the reaction zone, methanol or a carbonylatable derivative of methanol is carbonylated with carbon monoxide under pressure by simultaneous introduction of the 2 reactants. A given composition of the reaction medium is maintained by introducing the above two reactants and recycling streams originating from the 2nd and 3rd zones downstream. In the 2nd zone, called the vaporization zone or flash zone, the reaction medium is partially vaporized at a pressure below that in the 1st reaction zone, the expansion being effected with or without the provision of heat. The vaporized fraction, consisting mainly of the unreacted reactants, water, methyl iodide, methyl acetate and acetic acid, is transferred to the 3rd zone, called the zone for distillative separation and purification of the acetic acid and/or methyl acetate produced. The non-vaporized fraction coming from the 2nd flash zone, which is composed essentially of acetic acid and catalyst, is recycled into the 1st reaction zone. The 3rd zone, which conventionally consists of 3 distillation columns, makes it possible to separate the different components, to recycle those necessary for carrying out the reaction, and to produce purified acetic acid and/or methyl acetate. These processes also comprise a section for treatment of the gases and/or vents. For further details, reference may be made to the article by M. J. Howard et al., CATALYSIS TODAY, 18 (1993) 325–354.
Even more recently, carbonylation processes have been described which employ catalyst systems composed of both rhodium and iridium.
Patent EP 0 618 183, relating to the carbonylation of alcohols or derivatives, shows the synergism of a catalyst system composed of both rhodium and iridium compared with systems consisting of rhodium or iridium as the only metal. This process is carried out in the presence of small concentrations (0 to 5%, preferably 0 to 2%) of a catalyst stabilizer such as a soluble iodide salt, or in the absence of such compounds.
The soluble iodide salts consist of alkali metal or alkaline earth metal iodides or quaternary ammonium or phosphonium iodides.
Patent FR 2 750 984 describes the improvement in carbon monoxide consumption brought about by introducing a 2nd carbonylation reactor between the 1st zone (main reactor) and the 2nd vaporization (flash) zone; the catalyst system is composed of iridium, optionally in combination with rhodium.
Patent FR 2 750 985 relates to the carbonylation of methanol and proposes a method of stabilizing the catalyst system composed of soluble iridium and rhodium complexes, said method consisting in maintaining a water concentration of more than 0.5% in the non-vaporized liquid fraction recycled from the 2nd vaporization (flash) zone into the 1st reaction zone.
International patent application WO 00/27785 shows the synergism of a catalyst system composed of iridium and platinum, and optionally rhodium, compared with systems consisting of only one metal. Said patent application recommends maintaining a molar ratio of less than 10 between the soluble iodide salts introduced and the iridium.
International patent application WO 00/78700 relates to the production of acetic acid by means of simultaneous reactions involving the isomerization of methyl formate and the carbonylation of methanol in the presence of iridium and optionally rhodium. Said patent application teaches the stabilization of the catalyst system by maintaining the concentration of formyl radicals (methyl formate+formic acid) above 1% in the non-vaporized fraction from the 2nd vaporization (flash) zone. It is recommended to keep iodides in the soluble ionic form in this medium, although their concentration range is not stated.
International patent application WO 00/24701 describes the carbonylation of methanol with a catalyst system composed of both rhodium and iridium, in the presence of iodide ions as stabilizers/co-promoters of these catalysts. The composition of the reaction medium is kept constant: water <14% (preferably 0.1 to 8%), methyl iodide 5 to 30%, methyl acetate 0.5 to 30%, iodides (preferably lithium iodide) 2 to 20% and catalysts 100 to 5000 ppm for each metal; the catalyst system can also comprise a salt of a transition metal, preferably ruthenium.
The above patents and patent applications describe carbonylation reaction media established on the basis of a catalyst system which is composed of both rhodium and iridium and whose composition has been optimized to achieve the results expected of these inventions. None of these documents either indicates or suggests changing from a catalyst system composed of rhodium alone to a catalyst system based on a mixture of rhodium+iridium catalysts.
Furthermore, in such a change, it is well known that attention must be paid to the role of the corrosion metals commonly and conventionally present in carbonylation reaction media (iron, nickel, chromium, molybdenum, tungsten, zirconium and any metals which originate from the corrosion of industrial equipment and can be recycled into the reactor during production). As is apparent in particular in patent FR 2 551 434, these corrosion metals are generally considered overall to have detrimental effect in catalysis by rhodium alone, since they accelerate the conversion reaction or water gas reaction (also abbreviated to WGSR for Water Gas Shift Reaction):H2O+CO→H2+CO2
Again in catalysis by rhodium alone, patent EP 0 384 652, which is more selective, recommends removing iron and nickel in order to leave only the group VIb metals (Mo, W, Cr), which have a positive influence on the carbonylation rate and hence the productivity of the process. A variant consists in adding these group VIb corrosion metals to the carbonylation reaction medium.
On the other hand, in catalysis by iridium alone, the problem presented by the corrosion metals seems to be simpler: their concentration in the reaction medium must be limited to a few hundred ppm (patent FR 2 750 984) or to a value below 200 ppm (patent applications WO 00/27785 and WO 00/78700) in order to limit:                firstly the water gas reaction, as in the case of rhodium,        and secondly the concentration of iodide ions, which are considered to poison carbonylation catalyzed by iridium. In fact, these iodide ions, which originate from different sources, are introduced in part by the salts represented by the iodides of the corrosion metals. Patent FR 2 750 984 and patent application WO 00/27785 propose that the molar ratio of the iodide ions to the iridium be kept below 10.        
As can be seen from the foregoing, in view of the different influence (depending on the catalyst system) of both the concentration of corrosion metals and the concentration of iodide ions in the case of continuous processes for the manufacture of acetic acid by the carbonylation of methanol or one of its carbonylatable derivatives, the only solution hitherto envisaged by those skilled in the art wishing to change from a catalyst system based on rhodium to a catalyst system based on rhodium considerably enriched in iridium, or based on iridium alone, is to stop the installation in order to modify the reaction medium, in particular so as to adjust the concentration of corrosion metals and iodides to values which do not disturb the satisfactory running of the process with the new catalyst system.